Rotational molding involves a process for producing generally hollow, seamless plastic products. Its greatest use has been for thermoplastic materials in which a powder or liquid polymer is placed in a mold with the mold being heated and rotated simultaneously about two perpendicular axes, i.e., biaxially. Polyethylene or polypropylene are the most common thermoplastic materials in use.
When using a powdered material, the powder, in coming into contact with the heated rotating mold surface, melts and adheres thereto. As the mold continues to rotate the material continuously coats the heated mold surface to form a homogenous layer of uniform thickness. The mold is then cooled by air or water and opened to allow the part to be removed.
An enclosed volume, such as a hollow ring or void, for example, forming a rim of a container, can be created during the molding process by forming a narrow opening(s) between the main compartment and the void in the mold. Plastic flows through the opening and coats the interior of the space forming the void. Layers of the material continue to form on the walls of the mold defining the void until the opening(s) is closed as a result of the thickness of the plastic layers exceeding the width of the opening(s). At this time the opening is bridged with material continuing to flow inside of the mold section forming the main compartment.
One method of forming a void, in a rotationally molded refuse container, to provide a rim for the container, is described in U.S. Pat. No. 5,922,267 (“'267 patent”). In the '267 container the void is vented directly to atmosphere during the molding process via vent tubes extending from the enclosed void through the mold wall. These vents enable air to be expelled from the void during the later stages of the molding process to ensure the development of a uniform nonporous coating of material within the void. Alternatively, the air within the void may be vented to the interior of the mold via widened openings or gaps which are not bridged over. The air exiting the void as well as air within the main compartment is vented to the atmosphere via a vent tube extending through the mold wall. Such a venting gap(s) must have a width sufficiently large to avoid being bridged over by the layered material during the molding process.
The '267 patent is directed to a relatively small refuse container designed primarily for residential use. Larger waste containers for industrial and commercial use, e.g., having a capacity of several or more cubic yards, have traditionally been made of metal, i.e, steel. These waste containers, sometimes referred to as dumpsters, include side pockets or channels sized to receive the prongs or blades of a lifting mechanism, traditionally carried by waste transport trucks, which lift and empty the dumpster's contents into the truck's storage area for transport to a dump site. Such dumpsters conventionally have closure lids pivotally mounted to the back rim of the dumpster and many are equipped with a manual locking mechanism located at the front of the dumpster to prevent access to the dumpster's interior during certain times such as nights or weekends. In addition, casters are generally mounted to the bottoms of the dumpsters to facilitate their movement from a storage area to a location accessible by a waste transport truck. Such steel containers are not only heavy, but are subject to rusting, requiring considerable maintenance.
A prior art rotationally-molded plastic waste container, designed for industrial/commercial use, capable of holding several cubic yards of refuse is presently on the market. While the container is lighter and substantially maintenance free as compared to conventional steel dumpsters, it utilizes steel pockets for receiving the lifting mechanism prongs. The steel pockets which are bolted to the container side walls support the container and its contents on the lifting prongs. In addition, the lid is pivotally mounted to the back rim of the container by means of a steel bracket. The steel components are not only expensive as compared to the plastic material used to form the storage compartment, but must be manually attached to the plastic container during an assembly process and are subject to rusting. In addition, considerable noise is generated when the lifting prongs engage the steel pockets. In an effort to reduce this noise plastic sleeves have been inserted into the steel pockets increasing the costs. A brochure illustrating this prior art container, marketed by Nuwave Container, Inc., is attached to the Information Disclosure Statement (“IDS”). In addition, excerpts from the web sites of Cascade Engineering and Pacific Compactor Corp. Showing injection and rotationally molded trash containers in which steel pockets for receiving the lifting prongs are bolted to the container compartment are attached to the IDS.
There is a need for a molded plastic waste container which overcomes the above disadvantages.